I thought for coding tests implementations are supposed to be done from scratch on. Or at least the guy who wrote the working parser should be on top of the list.
Not really. We want people to solve the problem, not invent something that needs maintenance. In some cases, from scratch is important, but for most it's not.
No we don't want competent people showboating and pissing cash up the wall reinventing wheels that have a high maintenance cost because it's fun.
We hire engineers.
An analogy: would you hire the electrical engineer who knocks up an ASIC for every task or the one who designs in a COTS part for a predictable unit cost and manufacturing lead time?